It is well known that residues from the polymerization of alpha-olefin polymers using Ziegler-Natta catalysis show adverse effects on both stability and product color of films in which they are found. Increased amounts of stabilizers are needed to off set such deleterious effects from the catalyst residue. The adverse interactions typically result, in part, from the reactions of transition metal residues with phenolic antioxidants or their breakdown products. Chloride residues can react with the phenolic antioxidants catalyzing dealkylation and promoting autoxidation. Further, phenolate salts or coordination complexes can form that are chromophores.
The industry-wide approach has been to add metallic stearates of zinc, magnesium, and/or calcium as acid acceptors with the phenolic antioxidants. However, the use of metallic stearates can cause plateout or die lip buildup problems. They can also interfere with heat sealing and ink adhesion.
Certain polyols such as glycerol, sorbitol, or pentaerythritol, or their mono esters with fatty acids, have also been reported to give additional color improvements.
The polyols are not always found to have a positive effect on color improvement. The variable effectiveness of the polyols may be, in part, due to their solubility limitations and their inability to reach the catalyst residue.
Ultrafine zinc oxide or hydrotalcite (a hydrate of magnesium carbonate/aluminum hydroxide) can be used to replace the metallic stearates. However the use of these inorganic materials can give rise to dispersion problems. Moreover, such inorganic neutralizers by themselves or when wetted with small amounts of metallic stearate are not effective in controlling discoloration.
It is therefore an object of the present invention to provide a polyolefin composition which exhibits stability against the deleterious effects normally caused by catalyst residues present therein.
It is further an object of the present invention to provide a polyolefin composition which exhibits improved optical properties.
It has been found that the objects of the present invention are achieved by incorporation into polyolefin compositions of specific polyorganosiloxanes bearing hydroxyl functionality.
The use of polyorganosiloxanes as additives in polymer compositions is generally known. For instance, U.S Pat. No. 3,879,491 discloses the production of thermoplastic compositions containing from 1 to 15 weight percent of a hydroxy end-blocked polydiorganosiloxane. U.S. Pat. No. 4,430,235 discloses polydiorganosiloxane-based polymeric antioxidants, such as ##STR1## wherein Me represents a methyl group. U.S. Pat. No. 4,535,113 discloses polyolefin compositions having improved optical properties containing polydiorganosiloxanes bearing ethylene oxide, vicinal epoxy, or amino functionalities.